Thermal analysis (TA) refers to methods for measuring the temperature-dependant properties of materials. Both physical and chemical properties of materials may depend upon temperature. Examples of temperature-dependant materials properties include but are not limited to: glass transition temperature, melting temperature, magnetic coercivity, electrical resistance, electrical capacitance, index of refraction, solubility, and pH.
Usually, a material of interest may be a mixture, blend, or compound that has heterogeneity such that the temperature-dependence of one or more properties varies from location to location within the material. In this case, accurate TA measurements must measure spatially-resolved thermal properties in order to understand and tailor the overall material response. Novel TA techniques capable of high spatial resolution require instrumentation that is distinct from conventional TA, in that they must deliver heat and measure temperature at one specific location in the material of interest. The resolution of the TA technique is governed by the size of the heater and thermometer.
Conventional probes for high-resolution TA include a thin metal wire [1, 2]. These metal wire probes are bent and etched in order to form a tip. When an electrical current flows through the wire, heat is dissipated in the etched region of the wire. The wire has an electrical resistance that is a function of temperature, and thus it is possible to simultaneously measure both heating power and temperature in the tip. This metal wire probe can be mounted in an atomic force microscope (AFM) system [3]. The AFM system allows the probe tip to be precisely positioned in contact with a target substrate of interest. The AFM is able to measure the probe bending as it is pushing into the target substrate, and thus sample topography can be measured using the metal wire tip. Furthermore, the penetration of the tip into the substrate can also be measured as a function of probe tip temperature and heating power. However, the metal wire probe has one significant drawback, which is that the spatial resolution is in the range of 1 μm or greater. Thus the metal wire probe cannot be used to perform TA with a resolution much smaller than 1 μm.
For highly local TA, it would desirable to have a probe with a tip that is sharper than the conventional wire probe tip, and it would further be desirable to have a probe with an integrated heater-thermometer that is smaller than 1 μm.
In general, AFM probes with metal tips have not achieved nanometer-scale resolution, as it is difficult to fabricate electrically active metal probes that are extremely sharp [4-6]. However, silicon probes may be made to have tips with better than 10 nm sharpness, and that may make images with atomic-scale resolution [7].
Silicon probes made with integrated heater-thermometers have been demonstrated [8]. These silicon probes are originally designed for data storage [9]. They may be attractive for TA, as they have an extremely sharp tip and have an integrated heater-thermometer. However the heater-thermometer of the silicon probe is quite large at about 5×10 μm, and is not integrated into the end of the sharp tip but rather resides nearby. Thus, it would be desirable to have a sharp silicon probe tip having a heater-thermometer integrated into the tip, such that the size of the heater-thermometer is in the same or similar size to the sharpness of the probe tip, which however has not been available due to great technical barriers to be overcome.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.